(1) Field of Invention
The present invention relates to the fabrication of nanostructures and, more particularly, to a method of fabricating anchored carbon nanotube arrays into a thin polymeric layer.
(2) Description of Related Art
Solar energy is the only source of energy that can sustainably supply all of humanity's current and expected needs [5]. The primary limitations to its cost-effective use are efficiency in conversion of the available energy to a usable form, such as electricity, the modest energy density available, and infrastructure-related issues such as storage and transmission to mitigate the natural temporal variation in the energy source. Current developments in nanoscale fabrication and technology are showing promise for overcoming these limitations. Approaching the problem of harvesting solar energy, and more generally optical energy, at the nanoscale brings inherent increases in efficiency and decreases in packaging compared to large-scale approaches. Specially designed nanoscale materials are being aggressively pursued as solar cells and power sources [19].
Carbon nanotubes (CNTs) are a well-known nanoscale material, first reported in 1991 [11] and increasingly investigated ever since. CNTs themselves have not shown promise for solar electric application, that is the direct conversion of solar radiation to electricity, due to the fact that multi-walled CNTs behave as ballistic conductors [8], not semiconductors with a suitable bandgap for photovoltaic properties, and the fact that it is quite laborious to fabricate individual diodes which exhibit photovoltaic behavior from single-walled CNTs of the semiconducting variety [14]. This may change, however, as the ability to create monochiral multi-walled CNTs (where all walls within the CNTs have the same chirality) may soon be achieved [20]. These monochiral multi-walled CNTs may be easier to work with and also exhibit desired semiconductor behavior.
Additionally, highly selective growth methods for specific types of semiconducting single-walled CNTs and integration of large numbers of them into photovoltaic devices are under development. In any case, CNTs are promising for solar thermal applications, where solar energy is used for directly heating a working fluid which can then be used in various ways, such as electricity generation by conventional means (induction generators, in “high-temperature systems”) or simply as a heat source (so-called “mid-temperature systems”).
Substantially vertically aligned CNTs produced by molecular self-assembly in a chemical vapor deposition (CVD) chamber have very recently been shown to be the “darkest material” known to man [21], reflecting three times less light at several visible wavelengths than the previous record-holder. The initial measurements of CNTs in an anchored configuration have shown absorption of greater than about 99.9% of incident light in the continuous wavelength range of 270 nm to 2.6 μm, which includes the entire solar spectrum range, even the near-infrared. For comparison, state-of-the-art receiver coatings for high-temperature solar thermal systems, typically made of a multi-layer cermet structure, absorb about 96% of incident solar light. This extreme optical absorptivity of aligned arrays of CNTs within the solar spectrum makes them very promising as solar absorber materials, and represents a significant improvement over other approaches. There has not previously been a simple and straightforward method for making substantially vertically aligned CNT arrays usable for solar light collection. The previous limitation of having to use CNT arrays along with their growth substrates, usually silicon wafers, made it impractical to make industrial light collection devices based on CNTs. The present invention allows for the use of CNT arrays without their growth substrates, and thus solves the problem of practical use of CNTs for light collection.
A natural consequence of optical energy absorption is its conversion to heat. For example, strong absorption of near-infrared (700-1100 nm) radiation by single-walled CNTs has been used as a potential selective cancer therapy, where the localized heating of the CNTs, but not the surrounding tissue, killed only cells containing the CNTs [12]. In a related study, a modest amount of laser power (390 mW) at a near-infrared wavelength (1064 nm) results in over a 100° C. higher temperature in CNTs than in a carbonaceous graphite control sample [2], further evidencing that CNTs can get very hot by absorbing optical energy. Thus, solar optical energy can be efficiently converted to thermal energy by CNTs, and efficiently conducted along their length to be either utilized directly or transferred into a working fluid.
In addition to record-breaking optical properties, CNTs have some of the highest thermal conductivities known for any material at 650-3000 Wm−1K−1 [4] and extreme flexibility [6], which is an important property when considering usable lifetime and assembling industrial components. CNTscan be conveniently and scalably created by direct self-assembly in vertically aligned configurations on planar substrates [7]. The scalable fabrication means they can be produced in large quantities and in areas relevant to industry, without limitations such as being confined to the size of silicon wafers used in semiconductor manufacturing. Vertical alignment of the CNTs combined with their very high thermal conductivity indicates that heat will be efficiently and directionally conducted along their length into the rest of the system where the heat is utilized.
Therefore there exists a continuing need for a simple method of fabrication and use of anchored carbon nanotube array devices for integrated light collection and energy conversion.